VARIATION IN LIFE-HISTORY PATTERNS AMONG NEW-ZEALAND CHINOOK SALMON (ONCORHYNCHUS-TSHAWYTSCHA) POPULATIONS

Chinook salmon (Oncorhynchus tshawytscha) were introduced to New Zeala nd, apparently from a single California population, and have been self -sustaining since about 1905. Salmon from the Waimakariri, Rakaia, Ran gitata, and Waitaki rivers differed in various life history traits. Th e proportion of stream-type adults ranged from 29.1 to 75.6% in Rakaia and Waimakariri tributaries, respectively. Average age at maturity no t only differed among the major rivers but also between two tributarie s within the Rakaia River catchment. Length at age varied among rivers , the differences being most pronounced in older fish. Age structure a nd size at age combined to produce 91-, and 73-mm differences in mean length of males and females, respectively, among populations. Waitaki River salmon were not only long at age but also heavier for their leng th than other populations. The timing of entry into freshwater varied by over 1 mo between the Rakaia and Waitaki rivers, and median date of arrival on spawning grounds varied by 16 d between Rakaia and Waimaka riri river tributaries. These life history traits are influenced, to v arying degrees, by genetic and environmental factors. However, the sui te of differences indicates that considerable adaptation to local cond itions has occurred in about 20 generations.